I am building a flowmeter by measuring the speed of rotation of an impeller. The impeller I have produces a sine voltage and I would like to measure the time period of each individual wave i.e. the time between peaks. This is because the flow speed, and so, the frequency of the wave is constantly varying so I need to be able to take an average of all of the wave periods in a given time to calculate an average flow speed. Can anyone give me any ideas about how to go about this?